The objectives of this grant are to investigate the role of integrins in bone and to study the effect that glucocorticoids have on integrins. Integrins are a family of transmembrane receptors that mediate cell adhesion to specific extracellular matrix proteins (ECM). This interaction is fundamental for a cell to attach, migrate, proliferate and differentiate. We will concentrate on the integrins for collagen and fibronectin which are heterodimeric receptors, alpha2beta1 and alpha5beta1, respectively. We hypothesize that the downregulation of integrins by glucocorticoids has a profound effect on bone growth and contributes to the osteopenia apparent in osteoporosis. Our first aim is to determine the role of integrins in bone by incubating mineralizing organ cultures of fetal rat parietal bones with antibodies to integrins and assessing their effects on bone morphology and calcification. Glucocorticoid's effect on osteoblast organization, the polarity of integrins in the plasma membrane and collagen secretion will be studied. Our second aim is to demonstrate the effect of glucocorticoids on integrin protein levels in the plasma membrane and integrin mRNA expression in organ culture. The anabolic effect of insulin-like growth factor 1 (IGF-1) on integrins will also be studied. Experiments involving the treatment of rats with glucocorticoids will be performed to assess glucocorticoids effect on integrin levels in osteoblasts in vivo. The third goal is to determine the relationship between the synthesis of a specific ECM protein, Type 1 collagen, and its integrin, alpha2beta1. Ascorbic acid depletion and the addition of proline analogs will be used in organ culture to determine if they mimic the effect of glucocorticoids on integrins. Fluorescent activated cell sorting of osteoblasts will determine if osteoblasts with low levels of alpha2beta1 also have a decreased rate of collagen synthesis. The time course of glucocorticoid's effect on collagen and alpha2beta1 synthesis will also be examined. Knowledge of the role of integrins in the synthesis of ECM proteins and in mineralization and their regulation by hormones is primary to understanding bone growth and metabolic bone disease.